Hypothermia is a clinical entity defined as a core body temperature less than 35° C (95° F) and is classified in four stages depending on the temperature, symptomatology, and impact on specific organ physiology (Table 171.1).

All organs are ultimately affected by hypothermia, including the kidneys. The renal response to cold is rapid and varies with the different stages of hypothermia. Initially, peripheral vasoconstriction results in relative central hypervolemia, producing an increase in urine output. This response, termed cold diuresis, has been described even in patients with mild to moderate hypothermia. The etiology of this phenomenon is multifactorial and includes an initial increase in cardiac output and renal blood flow due to hypothermia-induced changes in vascular capacitance. Other important contributors are nonosmotic suppression of antidiuretic-hormone (ADH) release by the hypothalamus and subsequent decreased renal tubular reabsorption. These responses usually begin as soon as the core body temperature reaches 35° C and become more pronounced until moderate hypothermia, when decreased renal blood flow and glomerular filtration rate (reduced 50% at 27° to 30° C) may lead to renal failure.